Thermal storage devices such as storage heaters are well known as a source of electric heating. Traditionally they have operated under the principle that energy can be provided to the heater during specific periods of the day, and that supplied energy can be released from the heater during different time periods. As shown in the example of FIG. 1, the heaters comprise a heat store 120 in the form of bricks or other materials such as ceramic which is located within a housing 100 of the storage heater. The heatable material 120 is then heated using an electric element 110 so as to increase the temperature of the heatable material. This stored heat is then released continuously through a process of both thermal radiation and convection and conduction. The speed of heat transfer may be increased through use of a damper and/or in conjunction with mechanical fans. The heater is designed to release heat and typically has no more than 20% heat retention. It will be appreciated that industry standards define a storage heater as having such a heat retention rate.
Traditionally the use of storage heaters is prevalent in areas where the electricity network operator provides a two-tariff electricity meter. This allows the heating of the storage heater to be effected during periods of low cost electricity—such as during the night when the overall load on the network is less than peak times. The heating of the storage heater during this off peak period has to be sufficient to allow the heater to provide continuous heating to the area within which it is located during the intervals between heating. Typically these intervals can be as much as 12 hours. In a typical known mode of operation the heat output from the storage heater adopts a curve such as that shown in FIG. 2, which does not match the user demand for heat. As the heat is output from the storage heater in a continuous process, it has one peak output—shown in the example of FIG. 2 as occurring about 0900 in the morning. After that, its capacity to provide heat reduces with the result that when the user requires additional heat later in the day, the heater does not have the capacity to provide that heat.
To compensate for this discrepancy many heater manufacturers provide additional capacity for storing heat in their heaters. In this way the heat output of the storage heater is designed to be greater than that actually required. This is typically achieved by heating the storage materials to temperatures of the order of 700° C. While this provides additional heating capacity later in the evening, there is also continued wastage of heat during the non-use periods. This can also result in excessive heating of the room and a waste of heat.
It will be understood that there is a direct relationship between the energy input and the heat output of a conventional static storage heater which means the user has very limited control of heat output, typically no more than 15% of the total heat output. This makes the heater relatively unresponsive to changing weather conditions and user needs.
Many of these problems are discussed in GB2384300. This patent describes how operation of a traditional storage heater may be supplemented by a secondary heat source such as a radiant element which can be utilised to supplement the output of the main heat source—the storage material.
In the context of water cylinders, these are typically used as a source of domestic hot water. The dimensions of the water cylinder are selected so as to provide an adequate volume of hot water to a user within a prescribed time period—typically a 24 hour window. The energy used to heat the water within these known cylinders comes from a variety of sources including electrical, gas or oil powered boilers. It is also known to provide such cylinders with a primary source of energy and then use a secondary source for specific actions such as a top-up or where the primary source fails or is deactivated. Again the heat provided to this cylinder is directly coupled to the heat output expected from the cylinder.